In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these higher device densities there have been, and continue to be, efforts toward scaling down device dimensions (e.g., at sub-micron levels) on semiconductor wafers. In order to accomplish such densities, smaller feature sizes and more precise feature shapes are required. This can include width and spacing of interconnecting lines, spacing and diameter of contact holes, and surface geometry, such as corners and edges, of various features. The dimensions of and between such small features can be referred to as critical dimensions (CDs). Reducing CDs and reproducing more accurate CDs facilitates achieving higher device densities.
High resolution lithographic processes can be used to achieve small features. In general, lithography refers to processes for pattern transfer between various media. In lithography for integrated circuit fabrication, a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The film is selectively exposed with radiation (e.g., optical light, x-ray, electron beam, . . . ) through an intervening master template (e.g., mask, reticle, . . . ) forming a particular pattern (e.g., patterned resist). Dependent upon coating type, exposed areas of the coating become either more or less soluble than unexposed areas in a particular solvent developer. More soluble areas are removed with the developer in a developing step, while less soluble areas remain on the silicon wafer to form a patterned coating. The pattern corresponds to either the image of the mask or its negative. The patterned resist is used in further processing of the silicon wafer.
Efforts to reduce CDs have included implementing various techniques in connection with the lithographic process, such as reducing exposure radiation wavelength (e.g., from 436 nm mercury g-line to 365 nm i-line to 248 nm DUV to 193 nm excimer laser), improving optical design, utilizing metrology techniques (e.g., scatterometry, scanning electron microscope (SEM)), etc. Immersion lithography is another technique that facilitates further reduction of CDs.
In immersion lithography, a gap between a substrate (e.g., wafer) and a final optical component (e.g., lens, scanner, stepper, . . . ) is filled with an immersion medium, which has a refractive index greater than the refractive index of air. Refractive index is defined as a ratio of speed of light in a vacuum to speed of light in a particular medium. Utilizing an immersion medium with a refractive index greater than that of air, which approximately equals 1, can increase numerical aperture, which is defined as a lens's ability to gather diffracted light and resolve fine details onto a wafer. Furthermore, utilization of an immersion medium can decrease an effective wavelength of an exposure radiation propagating within the immersion medium without changing exposure radiation, lasers, lens materials, etc.
Differences in temperature can be present within the immersion medium. Such temperature variations between different portions of the immersion medium can cause flowing of the immersion medium (e.g., via convection) between regions with disparate temperatures and thus, yield turbulence within the immersion medium. Turbulence can create bubbles in the immersion medium, change pressure of the immersion medium, etc., which can impact optical characteristics of the immersion medium (e.g., refractive index, photolithographic constant), vary an effective wavelength of exposure radiation propagating through the immersion medium employed in connection with immersion lithography, or reflect and/or deflect and/or refract the exposure radiation. Thus, turbulence associated with the immersion medium can impact efficiency of immersion lithography systems and can elevate the costs associated with immersion lithographic processes. Thus, there is a need for systems and methods that improve immersion lithography.